Recently, we reported that reduction of intracellular Cl ร concentration ([Cl ร ] i ) inhibited proliferation of MKN28 gastric cancer cells by diminishing the transition rate from G 1 to S cell-cycle phase through upregulation of p21, cyclin-dependent kinase inhibitor, in a p53-independent manner. However, it is still unknown how intracellular Cl ร regulates p21 expression level. In this study, we demonstrate that mitogen-activated protein kinases (MAPKs) are involved in the p21 upregulation and cell-cycle arrest induced by reduction of [Cl ร ] i . Culture of MKN28 cells in a low Cl ร medium significantly induced phosphorylation (activation) of MAPKs (ERK, p38, and JNK) and G 1 /S cell-cycle arrest. To clarify the involvement of MAPKs in p21 upregulation and cell growth inhibition in the low Cl ร medium, we studied effects of specific MAPKs inhibitors on p21 upregulation and G 1 /S cell-cycle arrest in MKN28 cells. Treatment with an inhibitor of p38 or JNK significantly suppressed p21 upregulation caused by culture in a low Cl ร medium and rescued MKN28 cells from the low Cl ร -induced G 1 cell-cycle arrest, whereas treatment with an ERK inhibitor had no significant effect on p21 expression or the growth of MKN28 cells in the low Cl ร medium. These results strongly suggest that the intracellular Cl ร affects the cell proliferation via activation of p38 and/or JNK cascades through upregulation of the cyclin-dependent kinase inhibitor (p21) in a p53-independent manner in MKN28 cells.